This invention relates to galloping oscillations in transmission and distribution lines. In particular, it is an improved apparatus for suppressing galloping oscillations in aerial transmission and distribution lines.
The term "galloping" is applied to describe a mode of oscillation that has been observed in aerial electrical transmission lines. It is a form of wind-induced motion in which the conductors oscillate vertically. The motion of any particular portion of such a conductor is typically elliptical, with a major axis that is vertical or inclined moderately from the vertical. Looking at a span of a transmission line that exhibits galloping, one typically sees a standing wave with one or two maxima or loops in the span. It is less common to observe three or more loops between adjacent supports or to observe a travelling wave that is reflected at a support, but this has been observed.
The phenomenon of galloping has been analyzed in a classic paper by Den Hartog entitled "Transmission Line Vibration Due to Sleet," AIEE Transactions, Vol. 51, 1932, pp. 1074-6. The analysis of Den Hartog shows that the buildup of ice on one side of a conductor of a transmission line produces an airfoil that is capable of developing lift in response to a component of wind that is perpendicular to the line. Under selected conditions of lift and wind speed, oscillations may develop that build up to peak amplitudes of as much as tens of feet. This may cause line-to-line short circuits, damage to conductors, supports, and associated hardware, or all of these. This analysis is summarized and more recent results are discussed in a book entitled Transmission Line Reference Book: Wind-Induced Conductor Motion, Electric Power Research Institute, EPRI Research Project 792, 1979.
A number of mechanisms have been developed to reduce galloping in overhead power transmission lines. One of the first patents that appears to be devoted to the effects of wind on conductors is Dyer, Jr., U.S. Pat. No. 2,321,021, entitled "Composite Electrical Conductor." In that patent it is noted that wind gives rise to excessive transverse vibrations in single-wire conductors. The '021 patent represents an early attempt to shape conductors to reduce the effects of vibrations.
A later attempt at dealing with galloping in transmission lines is Shealy, U.S. Pat. No. 3,659,038, entitled "High-Voltage Vibration Resistant Transmission Line and Conductors Therefor." This patent teaches a composite conductor that is made by wrapping outer wires about a twisted pair of inner cables. The resulting composite conductor is said to resist both aeolian vibration and galloping. Aeolian vibration is wind-induced motion of conductors that is generally higher in frequency and lower in amplitude than galloping.
An early example of an apparatus that is designed to reduce galloping is given in the patent to A. S. Richardson, Jr., U.S. Pat. No. 3,440,328, entitled "Means for Damping Vibrations of Power Transmission Lines." This patent teaches the so-called Richardson damper, or Windamper, a structure designed to be clamped to transmission lines to reduce galloping.
Another structure for dealing with galloping is taught by Liberman in U.S. Pat. No. 3,388,208, entitled "Overhead Transmission Line with Aerodynamic Damper for Suppressing Galloping," which is incorporated here by reference as if set forth fully herein. The structure taught by Liberman is in some ways similar to that of Kerimov and Zaltsberg, U.S. Pat. No. 3,992,566, entitled "Aerodynamic Aerial Conductor Vibration Damper," which is also incorporated by reference as if set forth fully herein. Each of the latter two patents teaches a vane of plastic or the like that is attached vertically to a conductor to change the aerodynamic properties of the conductor, whether or not the conductor has been subject to a buildup of ice.
The aerodynamic dampers of the two latter patents have been shown to be effective in reducing galloping but these are associated with certain disadvantages. The '566 patent teaches an interference fit between the damper and the conductor. Achieving such a fit represents a significant manufacturing complication. Also, installation of the damper of the '566 patent is accomplished by forcing an opening in the upper quadrant of the damper onto a conductor of a transmission line. This frequently results in damaged dampers as the relatively thin edge at the opening of the damper is caught and creased upon insertion.
A further disadvantage of dampers made according to the '566 patent is that these dampers may lack holding power, especially on stranded conductors. This may let the wind twist a damper about the conductor of the transmission line, reducing its effectiveness in damping the aerodynamic forces that lead to galloping. The lack of holding power is increased when any portion of the inside surface of the damper is not in contact with the conductor. This has been observed to occur along the bottom of the conductor.